Add regression coverage for metadata newline preservation

src/Build.UnitTests/Construction/XmlReaderWithoutLocation_Tests.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.IO;
 using System.Xml;
 using Microsoft.Build.Evaluation;

src/Build.UnitTests/XmlReaderExtension_Tests.cs

@@ -0,0 +1,86 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+using Microsoft.Build.Evaluation;
+using Shouldly;
+using Xunit;
+
+#nullable disable
+
+namespace Microsoft.Build.UnitTests
+{
+    public class XmlReaderExtension_Tests
+    {
+        private readonly ITestOutputHelper _output;
+
+        public XmlReaderExtension_Tests(ITestOutputHelper output)
+        {
+            _output = output;
+        }
+
+        [WindowsOnlyFact]
+        public void ItemMetadataPreservesCrLfWhenLoadedFromDisk_Regression()
+        {
+            using TestEnvironment env = TestEnvironment.Create(_output);
+
+            TransientTestFile projectFile = env.CreateFile("metadata-newlines.proj", GetMultilineMetadataProjectContents());
+            var pc = env.CreateProjectCollection();
+            Project project = pc.Collection.LoadProject(projectFile.Path);
+            string metadataValue = project.GetItems("I").Single().GetMetadataValue("M");
+
+            metadataValue.ShouldBe(string.Join(Environment.NewLine,
+            [
+                "multiple",
+                "lines",
+                "in",
+                "this",
+                "metadatum",
+            ]));
+        }
+
+        [WindowsOnlyFact]
+        public void ItemMetadataPreservesCrLfWhenLoadedReadOnly_Regression()
+        {
+            using TestEnvironment env = TestEnvironment.Create(_output);
+
+            TransientTestFile projectFile = env.CreateFile("metadata-newlines-readonly.proj", GetMultilineMetadataProjectContents());
+
+            using var projectCollection = new ProjectCollection(
+                new Dictionary<string, string>(),
+                loggers: null,
+                remoteLoggers: null,
+                ToolsetDefinitionLocations.Default,
+                maxNodeCount: 1,
+                onlyLogCriticalEvents: false,
+                loadProjectsReadOnly: true);
+
+            Project project = projectCollection.LoadProject(projectFile.Path);
+            string metadataValue = project.GetItems("I").Single().GetMetadataValue("M");
+
+            metadataValue.ShouldBe(string.Join(Environment.NewLine,
+            [
+                "multiple",
+                "lines",
+                "in",
+                "this",
+                "metadatum",
+            ]));
+        }
+
+        private static string GetMultilineMetadataProjectContents() => "<Project>\r\n" +
+            "  <ItemGroup>\r\n" +
+            "    <I Include=\"I\">\r\n" +
+            "      <M>multiple\r\n" +
+            "lines\r\n" +
+            "in\r\n" +
+            "this\r\n" +
+            "metadatum</M>\r\n" +
+            "    </I>\r\n" +
+            "  </ItemGroup>\r\n" +
+            "</Project>";
+    }
+}

src/Build/Xml/XmlReaderExtension.cs

@@ -87,9 +87,12 @@ private static XmlReader GetXmlReader(string file, StreamReader input, bool load
             string uri = new UriBuilder(Uri.UriSchemeFile, string.Empty) { Path = file }.ToString();
 
 
-            // Ignore loadAsReadOnly for now; using XmlReader.Create results in whitespace changes
-            // of attribute text, specifically newline removal.
-            // https://github.com/dotnet/msbuild/issues/4210
+            // loadAsReadOnly is currently ignored.
+            // Compatibility note: XmlReader.Create normalizes whitespace/newlines in ways that changed
+            // observed project values (for example, multiline Exec commands and metadata), breaking
+            // existing builds. We intentionally keep XmlTextReader behavior here to preserve those
+            // established semantics until a non-reflection, compatibility-safe replacement exists.
+            // Related history: #4210, #4213, #4083, #6232, #6669.
             XmlReader reader = new XmlTextReader(uri, input) { DtdProcessing = DtdProcessing.Ignore };
 
             reader.Read();